When Winston goes to the meeting at O'Brien's apartment, he learns a lot of things about the book. For one thing, he gets a copy of the book itself. He reads it and learns what it says. So any of the things it says could potentially be the answer.

Two other things that he learns could include:

He learns that the book actually exists. People in general do not know if the book actually exists, but now he knows that it does.

He learns that the book is "nearly indestructible." O'Brien says that people could reproduce the book from memory if all the copies were destroyed.